In recent years the development and improvement of radiation sensors has been receiving considerable attention. It has been commonly found that most radiation sensors which are directed primarily to radiation in the infrared range utilize expensive and technically complex methods for selectively choosing the desired band width of the radiation which is to be sensed by the detector.
With the advent of intrusion detectors which are becoming more and more popular, it has been found that the public is in need of a low cost detector which can provide the desired alarm and protection and still be within the reach of the pocketbook of the average citizen. Up to now the radiation sensors which have been utilized for this purpose have required exotic structures which cost a considerable amount of money to produce.
One of the present techniques employs a crystalline filter placed between the infrared source and the sensor. These crystalline filters are typically a germanium or silicon substrate upon which thin layers of other optical materials are deposited. The property of these thin layers and substrates are controlled to give the entire filter the desired band pass and band rejection characteristics desired. The difficulties with these types of filters are that the deposition of a thin layer of material, which is usually metallic, onto the necessary substrate is a costly and time consuming process. In addition, the substrate materials are fragile and difficult to handle with considerable breakage resulting during manufacture. In addition, the raw materials which are used in fabricating these filters are relatively costly.
Another technique which is used with radiation detectors is specially treated mirrors to concentrate the radiation and to reflect only the desired wavelengths of the radiated energy onto the sensor. At present, these filtering mirrors are made by depositing a selective reflective material onto the mirror surface. These mirrors have proven unsatisfactory because of the difficulty which has been found in developing satisfactory reflective coatings with the desired band pass and band rejection characteristics.
Another problem occurs when the ambient temperature surrounding the detector changes which can produce false output signals from the sensor which in turn can be misinterpreted. It is highly possible that these ambient changes can sometimes be larger than temperature changes from a desired signal source. This condition, instead of producing false signals, can produce no detectable signal at all rendering the sensor unusable. It is a definite advantage that the sensor not respond to ambient temperature changes and still provide an output from desired radiated energy. Previously, various arrangements have been tried to minimize these conditions through the structure of the sensor itself or by compensation within the electronics which receives the output voltage signal from the sensor. An alternative that has been used in the past is to mount the sensor on a heat sink having a large mass to prevent rapid or sudden temperature changes.
As can be seen, many problems have occurred in the past with these types of sensors and the improvements and modifications which have been made up to this point have been primarily intended to overcome or minimize some of these recurring problems.